Conventional automotive vehicles include a powertrain (sometimes referred to as a drivetrain) that is generally comprised of an engine, multi-speed transmission, and a final drive (e.g., driveshaft, differential, and wheels). Hybrid type powertrains generally employ an internal combustion engine (ICE) and one or more motor/generator units that operate either individually or in concert to propel the vehicle—e.g., power output from the engine and motor/generators are transferred through planetary gearing in the multi-speed transmission to be transmitted to the vehicle's final drive. The electric-only drive capability of the vehicle is optimal where noise and/or exhaust emissions are of prime concern, whereas the engine-only drive capability is optimal where power output requirements exceed that of the motor/generator assembly.
Vehicles employing a hybrid powertrain (identified collectively as hybrid vehicles) are well suited for urban transportation where a significant amount of stop-and-go driving is undertaken. During urban travel, the hybrid powertrain takes advantage of both the ICE and motor/generator to improve upon fuel economy and exhaust emissions. For example, many hybrid powertrains permit the engine to be shut-off completely at vehicle stops to reduce fuel consumption. Additionally, the electric drive in some hybrid powertrains can be used for engine restart and/or vehicle acceleration. Moreover, some hybrid vehicles use electrical regenerative braking to recharge an internal electrical power storage device (i.e., batteries or similar component).
During regenerative braking, an onboard controller, such as a central processing unit (CPU) or electronic control unit (ECU), monitors the hybrid powertrain. When the vehicle is coasting downhill and/or the vehicle brakes are being applied, the controller reverses the polarity of the motor/generators, which resists rotation of the wheels, thereby providing a braking force. Effectively, the forward momentum of the vehicle (kinetic energy) is converted into electrical energy (e.g., through electromagnetic induction), which is then transferred to a battery pack for storage. The electrical energy produced by regenerative braking can thereafter be used to propel the vehicle or power vehicle accessories, providing even further improvements in fuel economy.
Hybrid powertrains are also well suited for over-the-road transportation, such as highway driving, where the electric motor/generator units can be utilized to assist in driving the vehicle during high-power output conditions such as rapid acceleration and hill climbing. The electric motor/generator units are also capable of providing propulsion in the event that engine operation is discontinued.
While hybrid vehicles offer the potential for significant fuel economy improvements over their conventional counterparts, their market penetration has been limited due to their relatively high cost/benefit ratio. It becomes pertinent to develop hybrid technologies that reduce cost and improve vehicle fuel economy. Two major contributors to the cost of hybrid vehicles are the capacity and complexity of certain hybrid powertrain componentry, and the size and number of motor/generators required to realize certain system requirements.